The invention relates to a fault simulation method and a fault simulator which prepare a fault list occurring in a semiconductor integrated circuit which can be detected with a test pattern sequence.
According to a conventional practice, a fault simulation of a semiconductor integrated circuit comprises assuming a fault within the integrated circuit, calculating an output value from an output terminal in response to a given test pattern by a logic simulation, determining a fault in which a change occurs as compared with an output value of a fault-free arrangement, assembling such results into a table of correspondence between assumed faults and input/output logic values, which is commonly referred to as a fault dictionary, thus preparing a fault list which can be detected with individual test patterns. When testing a semiconductor integrated circuit, each test pattern is applied to the input of the semiconductor integrated circuit, and the resulting output value and the input are used in making a reference to the fault dictionary in order to estimate whether or not there is a fault in the integrated circuit or/and where in the integrated circuit the fault is located.
In order to accommodate for a fault which does not give rise to a wrong logic such as a short-circuit fault or a current leak fault, there is proposed a fault simulation method which combines the logic simulation with IDDQ (quiescent power supply current) testing technique. According to this method, a logic signal value on a signal line within the integrated circuit which would occur in response to a test pattern is calculated by the logic simulation. By assuming faults within the integrated circuit to enumerate a variety of faults which satisfy requirements for the occurrence of an abnormality in IDDQ, a list of detectable faults is prepared. When a test pattern is input to the integrated circuit, each signal line in the integrated circuit normally assumes either xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d logic value. Accordingly, if a short-circuit fault occurs between a signal line having a logic value of xe2x80x9c0xe2x80x9d and a signal line having a logic value of xe2x80x9c1xe2x80x9d in the integrated circuit in response to the application of the test patter, an IDDQ abnormality occurs in the integrated circuit. In this manner, for any combination of a signal line having a logic value of xe2x80x9c0xe2x80x9d and a signal line having a logic value of xe2x80x9c1xe2x80x9d in the integrated circuit, a short-circuit fault which occurs between the both kinds of signal lines can be detected by the IDDQ testing technique in response to the input test pattern. When these short-circuit faults are enumerated, a fault list which can be detected with the IDDQ testing technique can be prepared.
However, the fault simulation method which employs the logic simulation can only deal with a fault model in which a signal line is fixed to a given state (either xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d, or a single stuck-at fault, namely stuck-at 0 or stuck-at 1). This prevents a sensitive simulation of a multiple stuck-at fault in which a plurality of signal lines are fixed to either xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d,a delay fault, a short-circuit fault between signal lines or the like. Accordingly, this fault simulation method cannot prepare a list of detectable faults for these faults.
Another difficulty of the fault simulation method which combines the logic simulation with the IDDQ testing technique is the fact that it is incapable of preparing a list of detectable faults for those faults related to a transient phenomenon of a semiconductor integrated circuit such as a delay fault or an open fault which influences upon a delay time or an abnormality in a local or global process parameter (such as sheet resistance, an oxide film thickness of the like) for example, inasmuch as the IDDQ testing technique is directed to determining a power supply current in the stable condition of the semiconductor integrated circuit or is principally directed to short-circuit faults in the circuit.
Accordingly, there is a need for a fault simulation method capable of preparing a fault list which are detectable with a test pattern sequence for faults including a delay fault, an open fault or a parametric abnormality fault in an integrated circuit.
It is an object of the invention to provide a fault simulation method and a fault simulator capable of preparing a fault list which are detectable with test pattern sequence for delay faults, open faults and parametric abnormality faults in a semiconductor integrated circuit, by using the IDDT (transient power supply current) testing technique which affords a high level of observability and capable of testing transient phenomena in the circuit in combination with a transition simulation.
According to the present invention, there is provided a method of preparing a fault list which are detectable with input test pattern sequence, comprising a step of deriving a test pattern sequence formed by two or more test patterns and which is to be applied to a semiconductor integrated circuit under test, a step of performing a transition simulation of an operation of the semiconductor integrated circuit under test when each test pattern in the derived test pattern sequence is applied thereto to calculate a train of transition signal values occurring on signal lines within the semiconductor integrated circuit under test, and a step of preparing a fault list which can be detected by the transient power supply current testing which uses the test pattern sequence, by utilizing the train of transition signal values on signal lines which are calculated by the transition simulation.
With this method, it is possible to prepare a fault list which are detectable by the transient power supply current testing which uses given test patterns, for those faults which have been difficult to detect in the prior art, namely, delay faults and open faults which give rise to delay faults, thus allowing a substantial improvement in the efficiency of testing against delay faults and open faults.
In one form of the step of preparing the fault list, the fault list is prepared in unit of a delay fault of a logic gate.
In another form of the step of preparing the fault list, the fault list is prepared in unit of an open fault in a signal line.
In a further form of the step of preparing the fault list, the fault list is prepared in unit of a path delay fault on a signal transmission path.
The present invention also provides a fault simulator which prepares a fault list which are detectable with an input test pattern sequence, comprising a test pattern sequence selector for deriving a test pattern sequence formed by two or more test patterns and which is to be applied to a semiconductor integrated circuit under test, a transition simulator which receives the derived test pattern sequence and performs a transition simulation of the operation of the semiconductor integrated circuit under test when each test pattern of the sequence is input thereto to calculate a train of transition signal values occurring on signal lines within the semiconductor integrated circuit under test, and a fault list preparing unit for preparing a fault list which are detectable by the transient power supply current testing which uses the test pattern sequence, by utilizing the train of transition signal values on signal lines which are calculated by the transition simulator.
The fault simulator allows a fault list which are detectable by the transient power supply current testing which uses certain test pattern to be prepared, for a delay fault or an open fault which leads to a delay fault, either of which has been difficult in the prior art to detect, thus permitting the testing efficiency for the delay faults and the open faults to be significantly improved.
In the description to follow, the principle of the present invention will be described in terms of a CMOS integrated circuit which is a most common semiconductor integrated circuit.
Transient current of CMOS logic gate
FIG. 1 shows a transient response of a CMOS inverter shown in FIGS. 1c and d. The transient response has been determined with a circuit simulator. FIG. 1a shows a response of an output voltage VOUT with respect to an input voltage VIN in a transient condition, while FIG. 1b shows a response of a current IDD which flows from a power supply into the CMOS inverter. The current IDD is referred to as a transient current. When the input IN of the inverter transitions from xe2x80x9c1xe2x80x9d to xe2x80x9c0xe2x80x9d (see FIG. 1c), n-MOS and P-MOS are only momentarily turned on as long as the input voltage is higher than the threshold voltage of n-MOS and lower than the threshold voltage of P-MOS, whereby a short-circuit current Is flows from a power supply terminal TVD to a ground GND. An output OUT from an output signal line of the inverter then transitions from xe2x80x9c0xe2x80x9d to xe2x80x9c1xe2x80x9d, and accordingly, at the same time as the short-circuit current IS, a current IC which is associated with a capacitance charging of a parasitic capacitance Cload connected to the output signal line of the inverter flows from the power supply terminal TVD to the parasitic capacitance Cload. Accordingly, when a falling transition (denoted by suffix xe2x80x9cfxe2x80x9d) occurs on the input IN of the inverter, the transient current IGf which flows into the inverter is given by the sum of the short-circuit current ISf and the capacitance charging current IC.
IGf=ISf+IC xe2x80x83xe2x80x83(1)
On the other hand, when the input IN transitions from xe2x80x9c0xe2x80x9d to xe2x80x9c1xe2x80x9d or when the output transitions from xe2x80x9c1xe2x80x9d to xe2x80x9c0xe2x80x9d (denoted by suffix xe2x80x9crxe2x80x9d) (see FIG. 1d), a capacitance discharge occurs from the parasitic capacitance Cload connected to the output signal line, thus producing a capacitance discharge current ID, but the current IGr which flows from the power supply terminal TVD to the inverter comprises only the short-circuit current ISr. Accordingly, a peak of the current TVD is slightly smaller than the peak of the transient current IGf which occurs during the falling transition as shown in FIG. 1b. 
IGr=ISr xe2x80x83xe2x80x83(2)
The CMOS inverter has a transfer characteristic which is such that a triangular pulse current IS is produced in response to a change in the input voltage VIN, as shown in FIG. 2a. Accordingly, when a rising transition occurs on the input of the CMOS inverter, the short-circuit current waveform ISr which flow through the CMOS inverter can be approximated by a triangular pulse, as indicated by xe2x80x9cISxe2x80x9d in FIG. 2b, assuming a ramp transition for the input voltage VIN. The short-circuit current waveform ISr with respect to the rising transition of the input signal as indicated in FIG. 2b can be given by the following approximations:                               I          Sr                =                  {                                                                      0                  ,                                                                              t                  ≤                                                                                    V                        THN                                                                    V                        DD                                                              ⁢                                          t                      r                                                                                                                                                                                                                                                                    V                            DD                                                    ·                                                      I                                                          S                              ⁢                                                              xe2x80x83                                                            ⁢                              max                                                                                                                                                            (                                                                                          V                                SP                                                            -                                                              V                                THN                                                                                      )                                                    ·                                                      t                            r                                                                                              ⁢                      t                                        -                                                                                            V                          THN                                                ·                                                  I                                                      S                            ⁢                                                          xe2x80x83                                                        ⁢                            max                                                                                                                      (                                                                              V                            SP                                                    -                                                      V                            THN                                                                          )                                                                              ,                                                                                                                                                V                        THN                                                                    V                        DD                                                              ⁢                                          t                      r                                                         less than                   t                  ≤                                                                                    V                        SP                                                                    V                        DD                                                              ⁢                                          t                      r                                                                                                                                                                                                                                                                    V                            DD                                                    ·                                                      I                                                          S                              ⁢                                                              xe2x80x83                                                            ⁢                              max                                                                                                                                                            (                                                                                          V                                SP                                                            -                                                              V                                DD                                                            +                                                              V                                THP                                                                                      )                                                    ·                                                      t                            r                                                                                              ⁢                      t                                        -                                                                                            (                                                                                    V                              DD                                                        -                                                          V                              THP                                                                                )                                                ·                                                  I                                                      S                            ⁢                                                          xe2x80x83                                                        ⁢                            max                                                                                                                      (                                                                              V                            SP                                                    -                                                      V                            DD                                                    +                                                      V                            THP                                                                          )                                                                              ,                                                                                                                                                V                        SP                                                                    V                        DD                                                              ⁢                                          t                      r                                                         less than                   t                  ≤                                                                                                              V                          DD                                                -                                                  V                          THP                                                                                            V                        DD                                                              ⁢                                          t                      r                                                                                                                                            0                  ,                                                                              t                  ≥                                                                                                              V                          DD                                                -                                                  V                          THP                                                                                            V                        DD                                                              ⁢                                          t                      r                                                                                                                              (        3        )            
where ISmax represents a maximum value of the transient current (short-circuit current) which flows into the CMOS inverter, VDD represents a power supply voltage, VTHN a threshold voltage of n-MOS transistor, VTHP a threshold voltage of a P-MOS transistor and tr a time interval for the rising transition of the input signal. It is to be noted that VTHP is indicated in its absolute magnitude. To simplify the approximations, it is assumed that the transition of the input voltage VIN begins at time 0, and the input voltage reaches to VDD at time tr which represents the completion of the transition.
Similarly, the short-circuit current waveform ISf of the CMOS inverter in response to the falling transition of the input signal can be given by the following approximations:                               I          Sf                =                  {                                                                      0                  ,                                                                              t                  ≤                                                                                    V                        THP                                                                    V                        DD                                                              ⁢                                          t                      f                                                                                                                                                                                                                                                                    V                            DD                                                    ·                                                      I                                                          S                              ⁢                                                              xe2x80x83                                                            ⁢                              max                                                                                                                                                            (                                                                                          V                                DD                                                            -                                                              V                                THP                                                            -                                                              V                                SP                                                                                      )                                                    ·                                                      t                            f                                                                                              ⁢                      t                                        -                                                                                            V                          THP                                                ·                                                  I                                                      S                            ⁢                                                          xe2x80x83                                                        ⁢                            max                                                                                                                      (                                                                              V                            DD                                                    -                                                      V                            THP                                                    -                                                      V                            SP                                                                          )                                                                              ,                                                                                                                                                V                        THP                                                                    V                        DD                                                              ⁢                                          t                      f                                                         less than                   t                  ≤                                                                                                              V                          DD                                                -                                                  V                          SP                                                                                            V                        DD                                                              ⁢                                          t                      f                                                                                                                                                                                                                                                                    V                            DD                                                    ·                                                      I                                                          S                              ⁢                                                              xe2x80x83                                                            ⁢                              max                                                                                                                                                            (                                                                                          V                                THN                                                            -                                                              V                                SP                                                                                      )                                                    ·                                                      t                            f                                                                                              ⁢                      t                                        -                                                                                            (                                                                                    V                              DD                                                        -                                                          V                              THN                                                                                )                                                ·                                                  I                                                      S                            ⁢                                                          xe2x80x83                                                        ⁢                            max                                                                                                                      (                                                                              V                            THN                                                    -                                                      V                            SP                                                                          )                                                                              ,                                                                                                                                                                          V                          DD                                                -                                                  V                          SP                                                                                            V                        DD                                                              ⁢                                          t                      f                                                         less than                   t                  ≤                                                                                                              V                          DD                                                -                                                  V                          THN                                                                                            V                        DD                                                              ⁢                                          t                      f                                                                                                                                            0                  ,                                                                              t                  ≥                                                                                                              V                          DD                                                -                                                  V                          THN                                                                                            V                        DD                                                              ⁢                                          t                      f                                                                                                                              (        4        )            
where tf represents a time interval for the falling transition of the input signal. For the sake of convenience, times for the start of the rising transition of the power supply current, the maximum value ISmax and the end of the falling transition are indicated in parentheses in FIG. 2b. 
Denoting a voltage change on the output signal line by vout (t), the charging current IC to the parasitic capacitance Cload connected to the output signal line of the CMOS inverter is given by the following equation:                               I          C                =                              C            load                    ⁢                                    ⅆ                                                v                  out                                ⁡                                  (                  t                  )                                                                    ⅆ              t                                                          (        5        )            
Such equations can be similarly determined for logic gates other than the inverter.
Assuming that the transient current IG which flows into a logic gate is substantially short-circuit current, it can be approximated by a triangular pulse as indicated by Is in FIG. 2b. In actuality, the transient current waveform IG of the CMOS inverter represents a triangular pulse as shown in FIG. 1b. Accordingly, the transient current IG of the logic gate increases monotonously until the maximum value ISmax is reached whereupon it decreases monotonously, as shown in FIG. 2b. The transient current IG reaches its maximum value ISmax when the input voltage VIN reaches the switching voltage VSP for the logic gate. Specifically, as shown in FIG. 2b, the time when the current IG reaches its peak coincides with the time of input transition of the logic gate. Because the logic gate has a delay time, the time for the output transition of the logic gate will be slightly lagging with respect to the time for the input transition. Thus, the time when the current IG reaches its peak slightly precedes the time for the output transition of the logic gate. In this instance, the falling edge of the transient current waveform IG may be considered as coincident with the time for the output transition. In addition, the transient current waveform IG of the logic gate has a pulse width which is proportional to the transition time of the input voltage (for example the time interval tr for the rising transition).
In the above description, it has been assumed that the transient current IG which flows into the logic gate is substantially a short-circuit current IS. However, with a miniaturization of a CMOS manufacturing process, the wiring delay will be more dominant over the gate delay. This means that assuming a constant transition time interval for the input voltage, the proportion of the charging current IC to the output signal line will be greater than the proportion of the short-circuit current IS in the transient current IG which flows into the CMOS logic gate. Accordingly, the time when the transient current waveform of the logic gate reaches its peak depends on the ratio of the charging current IC to the short-circuit current IS. When IC is less than IS, the peak of the transient current waveform IG coincides with the peak of IS. Because the peak of IS coincides with the transition time interval of the input voltage, it follows that the peak of IC precedes the time for the output transition of the logic gate. Conversely, if IC is greater than Is, the peak of the transient current waveform coincides with the peak of IC. Because the charging current IC relates to the voltage transition on the output signal line, the peak of IG is substantially coincident with the time for the output transition of the logic gate.
It will be seen from the equations (1), (2), (3) and (4) that the pulse width of the transient current waveform IG of the logic gate is proportional to the transition time of the input voltage (for example, the rising transition time tr). Here it is to be noted that because a change in the transition time of the output voltage is sufficiently small relative to a change in a transition time of the input voltage, IC term appearing in the equation (1) has been neglected.
Integral of transient power supply current
Using the equations (3) and (4), time integrals QSr and QSf of the short-circuit currents ISr and ISf are given as follows:                               Q          Sr                =                                            ∫                              -                ∞                            ∞                        ⁢                                          I                Sr                            ⁢                              ⅆ                t                                              =                                                                      I                                      S                    ⁢                                          xe2x80x83                                        ⁢                    max                                                  ⁡                                  (                                                            V                      DD                                        -                                          V                      THN                                        -                                          V                      THP                                                        )                                                            2                ⁢                                  V                  DD                                                      ⁢                          t              r                                                          (        6        )                                          Q          Sf                =                                            ∫                              -                ∞                            ∞                        ⁢                                          I                Sf                            ⁢                              ⅆ                t                                              =                                                                      I                                      S                    ⁢                                          xe2x80x83                                        ⁢                    max                                                  ⁡                                  (                                                            V                      DD                                        -                                          V                      THN                                        -                                          V                      THP                                                        )                                                            2                ⁢                                  V                  DD                                                      ⁢                          t              f                                                          (        7        )            
Accordingly, the integral QS of the short-circuit current through the logic gate as it is switched is given as follows:                                           Q            S                    ≡                                    ∫                              -                ∞                            ∞                        ⁢                                          I                S                            ⁢                              ⅆ                t                                                    =                                                                              I                                      S                    ⁢                                          xe2x80x83                                        ⁢                    max                                                  ⁡                                  (                                                            V                      DD                                        -                                          V                      THN                                        -                                          V                      THP                                                        )                                                            2                ⁢                                  V                  DD                                                      ⁢                          t              T                                ∝                      t            T                                              (        8        )            
where tT represents a transition time for the input signal. Thus, the integral QS of the short-circuit current IS (ISr or ISf) which flows into the logic gate is proportional to the time tT for the input transition of the logic gate. It will also be seen that QS does not depend on whether the transition of the input signal is rising or falling.
From the equation (5), it is seen that the integral QC of the charging current IC to an output load capacitance Cload of a CMOS inverter is given by the equation indicated below.                                                                         Q                C                            =                                                                    ∫                                          -                      ∞                                        ∞                                    ⁢                                                            I                      C                                        ⁢                                          ⅆ                      t                                                                      =                                                      ∫                                          -                      ∞                                        ∞                                    ⁢                                                            C                      load                                        ⁢                                                                  ⅆ                                                                              v                            out                                                    ⁡                                                      (                            t                            )                                                                                                                      ⅆ                        t                                                              ⁢                                          ⅆ                      t                                                                                                                                              =                                                                                          C                      load                                        ⁡                                          [                                                                        v                          out                                                ⁡                                                  (                          t                          )                                                                    ]                                                                            -                    ∞                                    ∞                                =                                                                            C                      load                                        ⁡                                          (                                                                        V                          DD                                                -                        0                                            )                                                        =                                                            C                      load                                        ⁢                                          V                      DD                                                                                                                              (        9        )            
and it is seen that it does not depend on the time tT for the input transition of the CMOS inverter.
Consequently, the integrals QGf and QGr of the transient currents IGf and IGr passing through the logic gate are determined from the equations (1), (2), (8) and (9) as follows:                                                                         Q                Gf                            =                              xe2x80x83                            ⁢                                                                    ∫                                          -                      ∞                                        ∞                                    ⁢                                                            (                                                                        I                          Sf                                                +                                                  I                          C                                                                    )                                        ⁢                                          ⅆ                      t                                                                      =                                                                                                                              I                                                      S                            ⁢                                                          xe2x80x83                                                        ⁢                            max                                                                          ⁡                                                  (                                                                                    V                              DD                                                        -                                                          V                              THN                                                        -                                                          V                              THP                                                                                )                                                                                            2                        ⁢                                                  V                          DD                                                                                      ⁢                                          t                      T                                                        +                                                                                                                        xe2x80x83                            ⁢                                                                    C                    load                                    ⁢                                      V                    DD                                                  ∝                                  t                  T                                                                                        (        10        )                                          Q                      G            r                          =                                            ∫                              -                ∞                            ∞                        ⁢                                          I                Sr                            ⁢                              ⅆ                t                                              =                                                                                          I                                          S                      ⁢                                              xe2x80x83                                            ⁢                      max                                                        ⁡                                      (                                                                  V                        DD                                            -                                              V                        THN                                            -                                              V                        THP                                                              )                                                                    2                  ⁢                                      V                    DD                                                              ⁢                              t                T                                      ∝                          t              T                                                          (        11        )            
Thus, an integral of the transient current through the logic gate is proportional to the time for the input transition of the logic gate.
Transient power supply current IDDI in CMOS integrated circuit
A power supply current in a CMOS integrated circuit represents a power supply current which flows into the CMOS integrated circuit, and is represented by a sum of currents which flow through individual logic gates within the integrated circuit.
A CMOS integrated circuit shown in FIG. 3a comprises a combination of four (G1, G2, G3, G4) of the inverter shown in FIG. 1c and which are connected in tandem. Transient currents IG1, IG2, IG3, IG4 which flow through the inverters G1, G2, G3, G4 are normally fed from a single power supply terminal TVD. Accordingly, the transient power supply current response of the integrated circuit is represented by a sum of transient currents flowing through the individual logic gates as shown in FIG. 3c and as given below.                               I          DDT                =                              ∑                          n              =              1                        N                    ⁢                      I            Gn                                              (        12        )            
where N represents the number of logic gates which are switched by input test pattern sequence. In the example shown in FIG. 3a, N=4. FIG. 3b shows a relationship between an input voltage and an output voltage of each logic gate.
Accordingly, when there is a fault in the integrated circuit which causes a change in the transient current of the logic gate and when there is a test pattern sequence which is capable of activating the fault, the fault can be detected by an observation of IDDT of the integrated circuit.
An integrated value QDDT of the transient power supply current IDDT can be represented as a sum of integrated values QGn (1xe2x89xa6nxe2x89xa6N) of currents which pass through individual logic gates.                                                                         Q                DDT                            =                                                                    ∫                                          -                      ∞                                        ∞                                    ⁢                                                            I                      DDT                                        ⁢                                          ⅆ                      t                                                                      =                                                      ∫                                          -                      ∞                                        ∞                                    ⁢                                                            (                                                                        ∑                                                      n                            =                            1                                                    N                                                ⁢                                                  I                          Gn                                                                    )                                        ⁢                                          ⅆ                      t                                                                                                                                              =                                                                    ∑                                          n                      =                      1                                        N                                    ⁢                                                            ∫                                              -                        ∞                                            ∞                                        ⁢                                                                  I                        Gn                                            ⁢                                              ⅆ                        t                                                                                            =                                                      ∑                                          n                      =                      1                                        N                                    ⁢                                      Q                    Gn                                                                                                          (        13        )            
In the example of FIG. 3a, the integrated value QDDT of the transient power supply current IDDT is represented as a sum of integrated values (QG1, QG2, QG3, QG4) of the currents (IG1, IG2, IG3, IG4) passing through the individual inverters. Accordingly, if there is a fault within the integrated circuit which causes a transient current through the logic gate or its time integral to change and if there is a test pattern which is capable of activating the fault, the fault can be detected from an observation of the integrated value QDDT of the transient power supply current IDDT of the integrated circuit.
Detection of gate delay fault
A gate delay fault is a fault which causes a signal propagation delay from an input to an output of a logic gate to increase. A fault which causes a rising transition of an output from the logic gate to retard is referred to as a slow-to-rise fault while a fault which causes a falling transition to retard is referred to as a slow-to-fall fault. When a gate delay fault occurs through a logic gate disposed within a CMOS integrated circuit, the output transition time of the logic gate is retarded. Accordingly, a logic gate which receives at its input an output from the faulty logic gate will have a retarded timing for the peak of the transient current. Accordingly, if a test pattern sequence could activate a faulty logic gate and switch another logic gate which is driven by the faulty gate, the gate delay fault can be detected from an observation of a lag in the current peak of IDDT of the CMOS integrated circuit.
Detection of open fault
Next an open fault which causes a delay fault will be defined. An open fault refers to an unintended electrical discontinuity, specifically indicating that a signal line is divided into two or more distinct signal lines. An open fault includes a breaking as may be caused by a poor contact due to an absence of metal or the presence of an oxide film, a breaking of a metal wiring due to poor patterning or etching, and a breaking in a diffusion layer or a polysilicon due to a mask failure. An open fault can be categorized into two types, including an open fault in which a wiring L is divided in two spaced portions L1 and L2, as shown in FIG. 4a and where an input VIN to one end of the wiring L cannot appear at an output VOUT at the other end of the wiring L to cause a xe2x80x9clogic faultxe2x80x9d as shown in FIG. 4b, and another open fault in which the divided wiling portions L1 and L2 are spaced very closely as shown in FIG. 4c and where an input signal VIN applied to one end of the wiring L flows as a tunnel current and a transition signal appears at the other end of the wiring L with a delay to cause a xe2x80x9cdelay faultxe2x80x9d as shown in FIG. 4d. An open fault which causes a logic fault involves a large scale of disconnection, so that there is no current flow if a voltage is applied across the signal lines (divided wiring portions L1 and L2) located on the opposite sides of the fault, and accordingly, there takes place no charging or discharge of the parasitic capacitance Cload in response to a signal transition, thus causing a logic fault in which the logic is fixed to a given value. By contrast in an open fault which causes a delay fault, a very weak current flows when a voltage is applied across the signal lines (L2 and L2) located on the opposite sides of the fault, but the magnitude of the current is less than a normal current flow, retarding the charging or discharge of the parasitic capacitance Cload in response to the signal transition, thus increasing a delay time of the circuit. An open fault which causes a delay fault includes a resistive open fault in which a resistance between signal lines L1 and L2 becomes larger than a normal value as a result of a poor contact or the resistance of the signal line L becomes larger than a normal value as a result of a failure occurring in the signal line L, and a minuscule open fault ( less than 100 nm) in which a very weak leak current flows across two broken signal line portions L1 and L2 by the tunnel effect. A tunnel current through a minuscule open fault is described, for example, in C. L. Henderson, J. M. Soden and C. F. Hawkins, xe2x80x9cThe Behavior and Testing Implications of CMOS IC Logic Gate Open Circuitsxe2x80x9d Proceedings of IEEE International Test Conference, pp.302-310, 1991. An open fault which causes a delay fault is dealt with herein and will be simply referred to as an open fault.
With the open fault, there is a small current flow through the fault, and thus it can be modeled by resistive element Ropen having an increased resistance. FIG. 5a shows an example of a CMOS integrated circuit having an open fault The CMOS integrated circuit comprises a pair of inverters G1, G2, and an open fault exists in a signal line M which connects between the inverters G1 and G2. The location of disconnection can be regarded as being equivalent to a connection through the resistor Ropen. When the inverter G1 switches to produce a signal transition on the signal line M, the presence of the open fault causes a time interval required for an input parasitic capacitor Cin of the inverter G2 to be charged or discharged increases (in proportion to the product of the resistance of the open fault and the parasitic capacitance), and accordingly, a signal transition on a signal line Mxe2x80x2 which is located on the inverter G2 side of the equivalent resistive element Ropen is retarded as shown in FIG. 5c relative to an output from the inverter G1 (FIG. 5b). Accordingly, as shown in FIG. 6, the transient current waveform (shown in dotted lines) of the inverter G2 having the open fault on its input side has a greater width of the current pulse and a greater time integral of the transient current as compared with the transient current waveform (shown in solid line) of the normal inverter. Accordingly, by observing IDDT or QDDT of the CMOS integrated circuit the presence of an open fault in the input stage of the inverter G2 can be detected. An open fault which exists on the output stage of the inverter G1 is equivalent to an open fault present on the input stage of the inverter G2, and thus can be similarly detected. For an open fault on any signal line in an integrated circuit generally, if an open fault can be activated by a test pattern sequence to cause a switching of a logic gate which is driven by the activated open fault, such open fault can be detected from an observation of IDDT or QDDT of the integrated circuit.
Path delay fault
A route within an integrated circuit through which a signal is propagated is referred to as a path. When a delay time from a start point of the path (input signal line) to an end point (output signal line), which is referred to herein as path delay time, becomes greater (or less) than a given value, the integrated circuit exhibits an abnormal behavior, which is referred to as a path delay fault. A path delay fault will now be defined.
Consider that for a CMOS logic circuit that a path P={g0, g1 . . . , gm} is activated using a test pattern sequence T= less than v1,v2 greater than  meaning that it contains two test patterns v1 and v2 and that a voltage signal v2 follows a voltage signal v1. g0 represents an input signal line of the path P while g1,g2, . . . , gm represent output signal lines of logic gates G1, G2, . . . , Gm on the path P. At the same time, g0,g1, . . . ,gmxe2x88x921 represent input signal lines to the logic gates G1,G2, . . . , Gm on the path P. Denoting the time for a signal transition on each signal line g0,g1. . . , gm (the time when the voltage signal passes VDD/2) by xcfx840,xcfx841, . . . xcfx84m, the gate delay time tgdt (1xe2x89xa6ixe2x89xa6m) of the respective logic gates G1,G2, . . . , Gm on the path P is given as follows:
tgdt=xcfx841xe2x88x92xcfx841xe2x88x92l xe2x80x83xe2x80x83(14)
Accordingly, the path delay time tpd of the path P is given as a sum of the gate delay times tgdi, as follows:                               t          pd                =                                            ∑                              i                =                1                            m                        ⁢                          t              gdi                                =                                    τ              m                        -                          τ              0                                                          (        15        )            
However, the actual gate delay time tgdt varies under the influence of a fault as follows:
tgdt=tgdt,typxcex41, 1xe2x89xa6ixe2x89xa6m xe2x80x83xe2x80x83(16)
where tgdt,typ represents a typical value of the gate delay time of the logic gate Gi, and xcex4i represents a variation component in the gate delay time. For example, a breaking fault causes the gate delay time of only a faulty logic gate to increase without increasing the delay time of other logic gates. A parametric fault causes the delay time of every logic gate to increase. With the variation in the gate delay time, the path delay time tpd similarly varies as indicated by an equation given below.                               t          pd                =                                            t                              pd                ,                typ                                      +            Δ                    =                                    ∑                              i                =                1                            m                        ⁢                          (                                                t                                      gdi                    ,                    typ                                                  +                                  δ                  i                                            )                                                          (        17        )            
where tpd,typ represents a typical value of a path delay time for the path P and xcex94 represents a variation component in the path delay time.
FIG. 7 is a schematic illustration of the fundamental principle of a delay fault testing technique. In order for a semiconductor integrated circuit (DUT) under test shown in FIG. 7a to operate normally, a signal transition which occurs in an input latch must pass through a path P in the semiconductor integrated circuit under test to be transmitted to an output latch in a given time interval. Accordingly, it will be seen from the relationship between an input VIN and an output VOUT and their relationship with respect to a system clock as shown in FIG. 7b, that the path delay time tpd of the path P must satisfy the following requirement:
tpd+TSU less than TCLKxe2x88x92TSKW xe2x80x83xe2x80x83(18)
where TSU represents a set-up time for the signal, TCLK. the period of the system clock and TSKW a clock skew of the system clock. The clock skew TSKW is a jitter of the system clock and represents an amount by which the edge of the system clock changes in the positive or negative direction. The equation (18) can be modified as follows:
tpd less than TCLKxe2x88x92TSKWxe2x88x92TSUxe2x89xa1Txe2x80x2xe2x80x83xe2x80x83(19)
Thus, the path delay time tpd of the path P must be less than a time interval Txe2x80x2 which results when margins such as the set-at time TSU and the clock skew TSKW are subtracted from the clock period TCLK. If tpd is greater than Txe2x80x2, the signal transmission along the path P cannot catch up with the system clock, and the circuit cannot operate properly. This condition is defined as a delay fault. In other words, the path P is defined as having a delay fault when tpd is greater than a given time interval Txe2x80x2. Txe2x80x2 represents an upper limit of a permissible delay time.
Detection of path delay (utilizing a pulse width of transient power supply current)
Since the peak or the falling edge of the transient current waveform of the logic gate corresponds to the time for the output transition of the logic gate, it follows that the last peak (or the last falling edge) of the transient power supply current waveform of the CMOS integrated circuit coincides with the time of the output transition of the logic gate which is the last to be switched in the CMOS integrated circuit Accordingly, by detecting the last peak (or the last falling edge) of the transient power supply current waveform of the CMOS integrated circuit, and comparing the time of detection against the time for the input transition, a path delay time for the integrated circuit can be determined. The time for the last falling edge of the transient power supply current can be determined as a maximum value of the time when the transient power supply current assumes a given current value, for example, from the time for the input transition on the path of the integrated circuit. This current value represents a value of the power supply current when the output voltage from the last logic gate on the path under test reaches a value equal to one-half the supply voltage VDD, and can be determined by the circuit simulation of the circuit under test or from statistical data obtained with an actual device.
A path delay fault on the path under test can be detected by comparing the determined path delay time against a given time (such as a period TCLK of a system clock, for example).
A technique for detecting a delay fault utilizing the pulse width of a transient power supply current mentioned above will now be described. According to this technique, the pulse width of the power supply current waveform of a circuit under test is measured and compared against a given time interval. The fundamental principle of the technique is illustrated in FIG. 8.
Assume that in a CMOS logic circuit, a plurality of paths P1, P2, . . . , Pn are activated using test pattern sequence T= less than v1, v2 greater than  including two test patterns v1, v2. Denoting the time when a j-th logic gate, as counted from the input of a path Pi, is switched by xcfx84ij, it will be noted that the number of logic gates depends on each of the paths P1,P2, . . . , Pn, and the time xcfx84max for the output transition of a logic gate Gfinal which is the last to be switched among the paths P1,P2, . . . , Pn is given by the following equation:                                           τ            max                    =                                    max                              i                ,                j                                      ⁢                          {                              τ                ij                            }                                      ,                  1          ≤          i          ≤          n                ,                  1          ≤          j                                    (        20        )            
Thus, a maximum value of path delay time tpd.max among the paths P1,P2, . . . , Pn is determined as a time interval between xcfx84max and time xcfx840 for the input transition, as indicated below.
tpd.max=xcfx84maxxe2x88x92xcfx840 xe2x80x83xe2x80x83(21)
On the other hand, the pulse width tPW of the transient power supply current waveform of the CMOS logic circuit is defined as a time interval between the time xcfx840 for the signal transition of the circuit input and the xcfx84IDD of the last peak (falling edge) of the transient power supply current waveform. Thus
tPWxe2x89xa1xcfx84IDDxe2x88x92xcfx840xe2x80x83xe2x80x83(22)
As mentioned previously, the time xcfx84IDD for the last peak of the transient power supply current waveform either coincides with the time xcfx84max for the output transition of the logic gate Gfinal which is the last to be switched or precedes xcfx84max. Thus, the pulse width tPW of the transient power supply current waveform corresponds to the delay time tpd.max of the path P which is activated by a test pattern T.
tPW=xcfx84IDDxe2x88x92xcfx840xe2x89xa6xcfx84maxxe2x88x92xcfx840=tpd.maxxe2x80x83xe2x80x83(23)
If tPW is greater than the upper limit Txe2x80x2 of the permissible delay time, it follows that
Txe2x80x2 less than tPWxe2x89xa6tpd max xe2x80x83xe2x80x83(24)
Thus, the signal transmission cannot catch up with the system clock on the path having the greatest delay time tpd.max. Thus, there exists a delay fault in the circuit. In this manner, tPW greater than Txe2x80x2 indicates the presence of a delay fault on some one of the activated paths, while tPW less than Txe2x80x2 indicates the absence of a delay fault on any activated path.
tPWxe2x89xa6Txe2x80x2 no delay fault 
tPW greater than Txe2x80x2 delay fault present xe2x80x83xe2x80x83(25)
In this manner, a delay fault in the circuit can be tested by comparing the pulse width tPW of the transient power supply current waveform against a given time Txe2x80x2.
Delay fault detecting technique (utilizing the instantaneous value of the transient power supply current)
Because the transient power supply current of a logic gate decreases monotonously after its peak as shown in FIG. 1b, it will be seen that the power supply current of the CMOS integrated circuit shown in FIG. 3c decreases monotonously subsequent to the time for the output transition of a logic gate which is the last to be switched in the integrated circuit. In other words, in a fault-free CMOS integrated circuit, denoting the time for the output transition of the last logic gate to be switched by xcfx84max and the instantaneous value of the transient power supply current at time xcfx84max by Ixe2x80x2, the transient power supply current of the CMOS integrated circuit cannot become greater than Ixe2x80x2 subsequent to xcfx84max.
Using this principle, a path delay fault in a circuit under test can be detected by measuring the instantaneous value of a transient power supply current of a CMOS integrated circuit at a given time. The current value Ixe2x80x2 which is used as a criterion for the judgment of a fault detection is a value of the power supply current at the time when an output from the last logic gate on a path under test assumes one-half the supply voltage, and can be determined by a circuit simulation of the circuit under test or from statistical data using an actual device.
A technique for detecting a delay fault utilizing the instantaneous value of the transient power supply current will be described. According to this technique, the instantaneous value of the transient power supply current of the circuit under test at a given time is measured, and is compared against the value of the transient power supply current of a golden circuit which is free from a delay fault. The fundamental principle of the technique is illustrated in FIG. 9.
Assume that in a CMOS logic circuit, a plurality of paths P1,P2, . . . ,Pn are activated by test pattern sequence T= less than v1,v2 greater than . Denoting the time when a j-th logic gate as counted from the input of a path Pi by xcfx84ij, the time xcfx84max for the output transition of a logic gate Gfinal which is the last to be switched among the paths P1,P2, . . . ,Pn is given by the following equation:                                           τ            max                    =                                    max                              i                ,                j                                      ⁢                          {                              τ                ij                            }                                      ,                  1          ≤          i          ≤          n                ,                  1          ≤          j                                    (        26        )            
Accordingly, a maximum value tpd.max of the path delay time among the paths P1,P2, . . . ,Pn can be determined as a time interval between xcfx84max and a time xcfx840 for the input transition, as indicated below.
tpd max=xcfx84maxxe2x88x92xcfx840 xe2x80x83xe2x80x83(27)
As mentioned previously, because the time for the output transition of a logic gate coincides with the time for the peak or the falling edge of the transient power supply current of the logic gate, it follows that xcfx84max corresponds to a time xcfx84IDD for the last peak or the falling edge of the transient power supply current waveform IDDT of the circuit. The power supply current IG of the logic gate can be approximated by a triangular wave and Gfinal represents the last gate to be switched, and hence, there is no logic circuit which has a peak of the power supply current subsequent to xcfx84max. Accordingly, a power supply current waveform function iDDT(t) is a monotonously decreasing function at time t which is txe2x89xa7xcfx84max. Thus, denoting the time function for the power supply current waveform by iDDT (t) and the instantaneous value of the power supply current at a time xcfx84max by Ixe2x80x2, the latter is given as follows:
Ixe2x80x2xe2x89xa1iDDT(xcfx84max) xe2x80x83xe2x80x83(28)
It follows that at time t which is txe2x89xa7xcfx84max,
iDDT(t)xe2x89xa6iDDT(xcfx84max)=Ixe2x80x2,txe2x89xa7xcfx84max xe2x80x83xe2x80x83(29)
In order for the circuit to operate properly, tpd.max must be less than the upper limit Txe2x80x2 of the delay time (which is equal to TCLKxe2x88x92TSKEW=TSU as indicated in the equation (19). Thus
tpd.max=xcfx84maxxe2x88x92xcfx840 less than Txe2x80x2xe2x80x83xe2x80x83(30)
Accordingly, in the absence of a fault in the circuit, it follows from the equation (29) that at time t which is t=Txe2x80x2+xcfx840 greater than xcfx84max 
iDDT(Txe2x80x2+xcfx840)xe2x89xa6Ixe2x80x2xe2x80x83xe2x80x83(31)
If the instantaneous value of IDDT at Txe2x80x2+xcfx840 is greater than Ixe2x80x2 or
iDDT(Txe2x80x2+xcfx840) greater than Ixe2x80x2=iDDT(xcfx84max) xe2x80x83xe2x80x83(32)
it follows from the equation (29) that because Txe2x80x2+xcfx840 cannot be greater than xcfx84max 
xcfx84max greater than Txe2x80x2+xcfx840 xe2x80x83xe2x80x83(33) 
tpd.max=xcfx84maxxe2x88x92xcfx840 greater than Txe2x80x2xe2x80x83xe2x80x83(34)
This means that on a path having the greatest delay time tpd.max , the signal transmission cannot catch up with the system clock. Thus the presence of a delay fault in the circuit is indicated. In is manner, the fact that a value of the transient power supply current IDDT (Txe2x80x2+xcfx840) at time Txe2x80x2+xcfx840 is greater than Ixe2x80x2 indicates the presence of a delay fault in one of the activated paths. Conversely, the fact that iDDT (Txe2x80x2+xcfx840) is less than Ixe2x80x2 indicates the absence of a delay fault on any activated path.
iDDT(Txe2x80x2+xcfx840)xe2x89xa6Ixe2x80x2 no delay fault 
iDDT(Txe2x80x2+xcfx840) greater than Ixe2x80x2 delay fault present xe2x80x83xe2x80x83(35)
As discussed above, a delay fault in the circuit can be detected by comparing the instantaneous value of IDDT at a given time against IDDT level of a fault-free circuit.
Detection of path delay fault (utilizing a time integral of transient power supply current
In addition, by using the integrated value QDDT of the transient power supply current IDDT, a path delay fault which is caused by an open fault can be detected. A technique for detecting a path delay fault by utilizing an integrated value of the transient power supply current will now be described. According to this technique, an integrated value of a transient power supply current through a circuit under test is measured, and is compared against a given value to evaluate a path delay fault.
The integrated value QDDT of the transient power supply current IDDT is represented as a sum of integrated values QGn(1xe2x89xa6nxe2x89xa6N) of currents passing through individual logic gates.                                                                         Q                DDT                            =                                                                    ∫                                          -                      ∞                                        ∞                                    ⁢                                                            I                      DDT                                        ⁢                                          ⅆ                      t                                                                      =                                                      ∫                                          -                      ∞                                        ∞                                    ⁢                                                            (                                                                        ∑                                                      n                            =                            1                                                    N                                                ⁢                                                  I                          Gn                                                                    )                                        ⁢                                          ⅆ                      t                                                                                                                                              =                                                                    ∑                                          n                      =                      1                                        N                                    ⁢                                                            ∫                                              -                        ∞                                            ∞                                        ⁢                                                                  I                        Gn                                            ⁢                                              ⅆ                        t                                                                                            =                                                      ∑                                          n                      =                      1                                        N                                    ⁢                                      Q                    Gn                                                                                                          (        13        )            
Since the integral QGn (1xe2x89xa6nxe2x89xa6N) of the current passing through each logic gate is proportional to the time tTn (1xe2x89xa6nxe2x89xa6N) for the input transition of each logic gate, as indicated by the equation (10) or (11), QDDT is given by a linear polynomial of tTn (1xe2x89xa6nxe2x89xa6N). In the example shown in FIG. 3a, QDDT is given by a linear polynomial 36, indicated below of the times (tT1,tT2,tT3,tT4) for the input transitions of the respective inverters (G1,G2 ,G3,G4 ).                               Q          DDT                =                                            ∑                              n                =                1                            N                        ⁢                          Q              Gn                                =                                                                      ∑                                      n                    =                    1                                    N                                ⁢                                  Q                  Sn                                            +                                                ∑                                      n                    =                    1                                    N                                ⁢                                  Q                  Cn                                                      =                                                            ∑                                      n                    =                    1                                    N                                ⁢                                                      a                    n                                    ⁢                                      t                    Tn                                                              +              b                                                          (        36        )            
In the equation (36), an represents a factor of proportionality between the integral QSn of the short-circuit current through the logic gate Gn and the time tTn for the input transition of the logic gate Gn, and b a constant term which is represented by a sum of charging currents QCn which flow into the respective logic gates.
An open fault can be modeled by a large resistance Ropen since a very weak current flows through the fault. FIG. 10a shows an example of CMOS inverter having an open fault at its input. When a signal transition as shown in FIG. 10b occurs on an input signal line A, a signal transition occurring on an signal line Axe2x80x2 which is located subsequent to the location of the fault is retarded due to the open fault as indicated in FIG. 10c. At this time, denoting the resistance of the open fault by Ropen and the parasitic capacitance at the input of the inverter by Cin, the time interval tT for the signal transition on the signal line Axe2x80x2 is given as indicated below
tT≈tT,typ+2.2RopenCin xe2x80x83xe2x80x83(37)
where tT.typ represents a typical value of the transition time for the input signal when there is no fault. The transition time tT is determined here as a time interval for the voltage to rise from 0.1 VDD to 0.9 VDD (or to fall from 0.9 VDD to 0.1 VDD) 2.2 Ropen Cin represents a time interval in order for a voltage across Cin rises from 0.1 VDD to 0.9 VDD, and is determined from loge (0.9 VDD/0.1 VDD )xc3x97Ropen Cm. Thus, an increment in the transition time for the input signal of the inverter is proportional to the resistance Ropen of the open fault. Accordingly, when there is an open fault on the input of a k-th inverter on the path under test the integral QDDT of the power supply current of the CMOS integrated circuit is determined from the equations (36) and (37) as indicated by an equation (38). Thus, QDDT linearly changes in accordance with the resistance Ropen of the open fault and the increment is proportional to the resistance Ropen of the open fault.                                                                         Q                DDT                            =                                                                                          ∑                                              n                        =                        1                                            N                                        ⁢                                                                  a                        n                                            ⁢                                              t                        Tn                                                                              +                  b                                =                                                      (                                                                                            ∑                                                      n                            =                            1                                                    N                                                ⁢                                                                              a                            n                                                    ⁢                                                      t                                                          Tn                              ,                              typ                                                                                                                          +                      b                                        )                                    +                                      2.2                    ⁢                                          a                      k                                        ⁢                                          C                      in                                        ⁢                                          R                      open                                                                                                                                              =                                                                    Q                                          DDT                      ,                      typ                                                        +                                      2.2                    ⁢                                          a                      k                                        ⁢                                          C                      in                                        ⁢                                          R                      open                                                                      ∝                                  R                  open                                                                                        (        38        )            
where QDDT typ represents a typical value of the integral of the power supply current when there is no fault. 2.2ak Cin Ropen appearing as a second term on the right hand side of the equation (38) is an addition which is based on the input open fault of the k-th inverter. The equation (38) shows a coincidence with a result of simulation performed on a change in QDDT with respect to Ropen as shown in FIG. 11. FIG. 11 is a plot of a change in QDDT with respect to the resistance Ropen of the open fault when there is an open fault in the input signal line IN2 of the inverter G2 in the circuit shown in FIG. 3.
A gate delay time tgd of a logic gate is proportional to the time tT for the transition of an input signal as indicated by an equation (39) below. ( see equations 4.52 and 4.53, xe2x80x9cPrinciples of CMOS VLSI Design-A Systems Perspectivexe2x80x9d, Second Edition, Addison-Weely Publishing Company, issued 1999, pp. 216-217)                               t          gd                =                              t                          gd              ,              step                                +                                    1              6                        ⁢                          (                              1                -                                  2                  ⁢                                                            V                      TH                                                              V                      DD                                                                                  )                        ⁢                          t              T                                                          (        39        )            
where tgd.step represents a delay time of a fault free inverter with respect to a step input at transition time 0, and VTH a threshold voltage for PMOS or n-MOS, it being understood that VTH=VTHN for a rising transition of an input, and VTH=VTHP for a falling transition of an input. Accordingly, since a gate delay time tgd of a logic gate having an open fault which can be modeled by a resistance Ropen on an input signal line is given by the equation (37), it can be determined by substitution of the equation (37) into the equation (39), as indicated below.                                                                         t                gd                            =                                                t                                      gd                    ,                    step                                                  +                                                                            t                      T                                        6                                    ⁢                                      (                                          1                      -                                              2                        ⁢                                                                              V                            TH                                                                                V                            DD                                                                                                                )                                                                                                                          =                                                t                                      gd                    ,                    step                                                  +                                                                                                    t                                                  T                          ,                          typ                                                                    +                                              2.2                        ⁢                                                  R                          open                                                ⁢                                                  C                          in                                                                                      6                                    ⁢                                      (                                          1                      -                                              2                        ⁢                                                                              V                            TH                                                                                V                            DD                                                                                                                )                                                                                                                          =                                                t                                      gd                    ,                    step                                                  +                                                                            t                                              T                        ,                        typ                                                              6                                    ⁢                                      (                                          1                      -                                              2                        ⁢                                                                              V                            TH                                                                                V                            DD                                                                                                                )                                                  +                                                                            2.2                      ⁢                                              C                        in                                                              6                                    ⁢                                      (                                          1                      -                                              2                        ⁢                                                                              V                            TH                                                                                V                            DD                                                                                                                )                                    ⁢                                      R                    open                                                                                                                          =                                                                    t                                          gd                      ,                      typ                                                        +                                                                                    2.2                        ⁢                                                  C                          in                                                                    6                                        ⁢                                          (                                              1                        -                                                  2                          ⁢                                                                                    V                              TH                                                                                      V                              DD                                                                                                                          )                                        ⁢                                          R                      open                                                                      ∝                                  R                  open                                                                                        (        40        )            
where tgd.typ is a typical value of the gate delay time of a fault-free logic gate. Specifically, the gate delay time tgd of a logic gate having an open fault varies depending on the resistance Ropen of the fault, and the increment xcex4of the gate delay time is proportional to the resistance Ropen of the fault. Consequently, when there is a braking fault on the input of any logic gate on the path under test, the path delay time tpd of the path under test is also proportional to Ropen. This is indicated by an equation (41) given below which is obtained by the substitution of the equation (40) into the equation (17).                                                                         t                pd                            =                                                ∑                                      i                    =                    1                                    m                                ⁢                                  t                  gdi                                                                                                        =                                                                    ∑                                          i                      =                      1                                        m                                    ⁢                                      t                                          gdi                      ,                      typ                                                                      +                                                                            2.2                      ⁢                                              C                        ink                                                              6                                    ⁢                                      (                                          1                      -                                              2                        ⁢                                                                              V                            TH                                                                                V                            DD                                                                                                                )                                    ⁢                                      R                    open                                                                                                                          =                                                                    t                                          pd                      ,                      typ                                                        +                                                                                    2.2                        ⁢                                                  C                          ink                                                                    6                                        ⁢                                          (                                              1                        -                                                  2                          ⁢                                                                                    V                              TH                                                                                      V                              DD                                                                                                                          )                                        ⁢                                          R                      open                                                                      ∝                                  R                  open                                                                                        (        41        )            
This agrees with the result of simulation conducted on a change of tpd with respect to Ropen as indicated in FIG. 12, which is a plot of a change of tpd with respect to the resistance Ropen of an open fault when there is an open fault in the input signal line IN2 of an inverter G2 in the circuit shown in FIG. 3a. 
When an open fault exist on the input of a logic gate Gk on the path P, the integral QSk of the short-circuit current of the gate Gk is determined from the equations (8) and (37) as given below                               Q          Sk                =                  xe2x80x83                ⁢                                                            I                                  S                  ⁢                                      xe2x80x83                                    ⁢                  max                                            ⁡                              (                                                      V                    DD                                    -                                      V                    THN                                    -                                      V                    THP                                                  )                                                    2              ⁢                              V                DD                                              ⁢                      t            Tk                                                  =                  xe2x80x83                ⁢                                                            I                                  S                  ⁢                                      xe2x80x83                                    ⁢                  max                                            ⁡                              (                                                      V                    DD                                    -                                      V                    THN                                    -                                      V                    THP                                                  )                                                    2              ⁢                              V                DD                                              ⁢                      (                                          t                                  Tk                  ,                  typ                                            +                              2.2                ⁢                                  R                  open                                ⁢                                  C                  ink                                                      )                                                  =                  xe2x80x83                ⁢                                                                              I                                      S                    ⁢                                          xe2x80x83                                        ⁢                    max                                                  ⁡                                  (                                                            V                      DD                                        -                                          V                      THN                                        -                                          V                      THP                                                        )                                                            2                ⁢                                  V                  DD                                                      ·                          t                              Tk                ,                typ                                              +                                                  xe2x80x83                ⁢                                                                              I                                      S                    ⁢                                          xe2x80x83                                        ⁢                    max                                                  ⁡                                  (                                                            V                      DD                                        -                                          V                      THN                                        -                                          V                      THP                                                        )                                                            2                ⁢                                  V                  DD                                                      ·            2.2                    ⁢                      R            open                    ⁢                      C            ink                                                  =                  xe2x80x83                ⁢                              Q                          Sk              ,              typ                                +                                                    2.2                ⁢                                                      I                                          S                      ⁢                                              xe2x80x83                                            ⁢                      max                                                        ⁡                                      (                                                                  V                        DD                                            -                                              V                        THN                                            -                                              V                        THP                                                              )                                                  ⁢                                  C                  ink                                                            2                ⁢                                  V                  DD                                                      ·                          R              open                                          
Accordingly, the integral QDDT of the transient power supply current of the integrated circuit is determined according to the equation (36) as given below.                                                                         Q                DDT                            =                              xe2x80x83                            ⁢                                                ∑                                      n                    =                    1                                    N                                ⁢                                  Q                  Gn                                                                                                        =                              xe2x80x83                            ⁢                                                                    ∑                                          n                      =                      1                                        N                                    ⁢                                      Q                    Sn                                                  +                                                      ∑                                          n                      =                      1                                        N                                    ⁢                                      Q                    Cn                                                                                                                          =                              xe2x80x83                            ⁢                                                                    ∑                                          n                      =                      k                                        N                                    ⁢                                      Q                                          Sn                      ,                      typ                                                                      +                                  Q                                      Sk                    ,                    typ                                                  +                                                                                                        xe2x80x83                            ⁢                                                                                          2.2                      ⁢                                              xe2x80x83                                            ⁢                                                                        I                                                      S                            ⁢                                                          xe2x80x83                                                        ⁢                            max                                                                          ⁡                                                  (                                                                                    V                              DD                                                        -                                                          V                              THN                                                        -                                                          V                              THP                                                                                )                                                                    ⁢                                              C                        ink                                                                                    2                      ⁢                                              V                        DD                                                                              ·                                      R                    open                                                  +                                                      ∑                                          n                      =                      k                                        N                                    ⁢                                      Q                    Cn                                                                                                                          =                              xe2x80x83                            ⁢                                                                    ∑                                          n                      =                      1                                        N                                    ⁢                                      Q                                          Sn                      ,                      typ                                                                      +                                                      ∑                                          n                      =                      1                                        N                                    ⁢                                      Q                    Cn                                                  +                                                                                                        xe2x80x83                            ⁢                                                                    2.2                    ⁢                                          xe2x80x83                                        ⁢                                                                  I                                                  S                          ⁢                                                      xe2x80x83                                                    ⁢                          max                                                                    ⁡                                              (                                                                              V                            DD                                                    -                                                      V                            THN                                                    -                                                      V                            THP                                                                          )                                                              ⁢                                          C                      ink                                                                            2                    ⁢                                          V                      DD                                                                      ·                                  R                  open                                                                                                        =                              xe2x80x83                            ⁢                                                Q                                      DDT                    ,                    typ                                                  +                                                                            2.2                      ⁢                                              xe2x80x83                                            ⁢                                                                        I                                                      S                            ⁢                                                          xe2x80x83                                                        ⁢                            max                                                                          ⁡                                                  (                                                                                    V                              DD                                                        -                                                          V                              THN                                                        -                                                          V                              THP                                                                                )                                                                    ⁢                                              C                        ink                                                                                    2                      ⁢                                              V                        DD                                                                              ·                                      R                    open                                                                                                          (        42        )            
Thus, the integral QDDT of the transient power supply current of the integrated circuit is also proportional to the resistance Ropen of the open fault.
Thus, it will be seen from the equations (41) and (42) that the delay time tpd of the path P having an open fault changes linearly with respect to the integral QDDT of the transient power supply current of the CMOS integrated circuit, and this agrees with the result of a simulation conducted on the change of tpd with respect to QDDT as indicated in FIG. 13, which is a plot of a change tpd with respect to the integral QDDT of the transient power supply current when an open fault is present on the input signal line IN2 of an inverter G2 in the circuit shown in FIG. 3a. 
The substitution of Ropen which is determined from the equation (42) into the equation (41) yields an equation (43).                                                                         t                pd                            =                              xe2x80x83                            ⁢                                                t                                      pd                    ,                    typ                                                  +                                                                            2.2                      ⁢                                              C                        ink                                                              6                                    ⁢                                      (                                          1                      -                                              2                        ⁢                                                                              V                            TH                                                                                V                            DD                                                                                                                )                                    ⁢                                      R                    open                                                                                                                                          xe2x80x83                            ⁢                                                t                                      pd                    ,                    typ                                                  +                                                                            2.2                      ⁢                                              C                        ink                                                              6                                    ⁢                                                            (                                              1                        -                                                  2                          ⁢                                                                                    V                              TH                                                                                      V                              DD                                                                                                                          )                                        ·                                                                                                                                          xe2x80x83                            ⁢                                                                                          (                                                                        Q                          DDT                                                -                                                  Q                                                      DDT                            ,                            typ                                                                                              )                                        ·                    2                                    ⁢                                      V                    DD                                                                    2.2                  ⁢                                                            I                                              S                        ⁢                                                  xe2x80x83                                                ⁢                        max                                                              ⁡                                          (                                                                        V                          DD                                                -                                                  V                          THN                                                -                                                  V                          THP                                                                    )                                                        ⁢                                      C                    ink                                                                                                                          =                              xe2x80x83                            ⁢                                                t                                      pd                    ,                    typ                                                  +                                                                                                    V                        DD                                            -                                              2                        ⁢                                                  V                          TH                                                                                                            3                      ⁢                                                                        I                                                      S                            ⁢                                                          xe2x80x83                                                        ⁢                            max                                                                          ⁡                                                  (                                                                                    V                              DD                                                        -                                                          V                              THN                                                        -                                                          V                              THP                                                                                )                                                                                                      ⁢                                      (                                                                  Q                        DDT                                            -                                                                                                                                                              xe2x80x83                            ⁢                              Q                                  DDT                  ,                  typ                                            )                                                          (        43        )            
Denoting the integral of the transient power supply current when the path delay time tpd is equal to the upper limit Txe2x80x2 of the permissible delay time by Qmax, an equation (44), given below, is obtained by solving the equation (43) for Qmax by putting tpd=Txe2x80x2 and QDDT=Qmax.                               Q          max                =                              Q                          DDT              ,              typ                                +                                                    3                ⁢                                                      I                                          S                      ⁢                                              xe2x80x83                                            ⁢                      max                                                        ⁡                                      (                                                                  V                        DD                                            -                                              V                        THN                                            -                                              V                        THP                                                              )                                                                                                V                  DD                                -                                  2                  ⁢                                      V                    TH                                                                        ⁢                          (                                                T                  xe2x80x2                                -                                  t                                      pd                    ,                    typ                                                              )                                                          (        44        )            
The Qmax represents the upper limit of the integral QDDT of the transient power supply current of the CMOS integrated circuit which is free from a delay fault. Thus it can be determined that there is no path delay fault in the CMOS integrated circuit when QDDT is less than Qmax while there exists a path delay fault due to a break fault in the CMOS integrated circuit when QDDT is greater than Qmax.
QDDTxe2x89xa6Qmax no delay fault 
QDDT greater than Qmax delay fault present xe2x80x83xe2x80x83(45)
As discussed above, by comparing the integral QDDT of the transient power supply current against the given value Qmax, a delay fault in the circuit can be tested The given value Qmax can be determined by a circuit simulation or from the equation (45) using statistical data.
Features of a delay fault test by way of the transient power supply current
The transient power supply current is a transient current which flows through a power supply pin of an integrated circuit, and a higher observability is assured for this transient current than for a voltage signal. Accordingly, a delay fault testing method which employs the transient power supply current is capable of assuring a high observability of a delay fault than with a delay fault testing technique which employs a voltage signal. By way of example, a delay fault testing technique which employs a voltage signal can not detect a delay fault unless the voltage signal is transmitted to an output signal line of an integrated circuit. By contrast with a delay fault testing technique which employs a transient power supply current signal, a transient power supply current signal having a pulse width which corresponds to a delay time on a path on which the voltage signal is transmitted is observable if the voltage signal is not transmitted to an output signal line of the integrated circuit, and thus is capable of detecting a delay fault. Concomitantly, there is no need to transmit a voltage signal to an output signal line of an integrated circuit with the delay fault testing technique which employs the transient power supply current signal and accordingly, limitations imposed upon generating test patterns are reduced with this technique in comparison to the generation of test patterns for a conventional delay fault testing technique which employs a voltage signal where it is necessary to transmit the voltage signal to the output signal line of the integrated circuit. This allows the generation of test patterns to be facilitated. In extreme cases, if test pattern sequence are selected at random, the delay fault testing technique which employs the transient power supply current signal is capable of detecting a delay fault on a path which is activated by selected test pattern sequence.
Fault simulation method (for gate delay fault)
The fault simulation method according to the present invention which prepares a fault list in unit of a gate delay fault will now be described. FIG. 14 shows an example of a MOS integrated circuit under test. The integrated circuit under test includes four input terminals x1, x2, x3, x4, two output terminals z1, z2, five internal signal nodes n1, n2, n3, n4, n5, five logic gates G1, G2, G3, G4, G5 and output buffers G6, G7. Input terminals x1 and x2 are connected to two inputs of NAND gate G1, the output of which is connected through the signal node n1, to the input of the inverter gate G2 and to one input of OR gate G3. The output from the inverter gate G2 is connected through the node n2 to one input of AND gate G4, and the output from the gate G3 is connected through the node n3 to the other input of AND gate G4 and to one input of AND gate G5. The input terminal x3 is connected to the other input of OR gate G3, and the input terminal x4 is connected to the other input of AND gate G5. Outputs from AND gates G4 and G5 are connected through the nodes n4 and n5, respectively, and further through the output buffers G6, G7 to the output terminals z1 and z2, respectively. The power supply terminals of the gates G1 to G7 are connected to a common power supply, while this is not shown.
Here it is assumed that the gates G1, G2, G3, G4, G5, G6 and G7 have gate delays (propagation delays) equal to 1, 1, 3, 2, 1, 1 and 1, respectively. An example of results of a fault simulation which is performed upon the MOS integrated circuit under test is illustrated in FIG. 15. In FIG. 15, the first column, as counted from the left, represents an identifier of a test pattern sequence; the second column shows input signals applied to the input terminals x1, x2, x3, x4 of the CMOS integrated circuit under test; the third column shows a train of transition signal values which appear on the internal signal nodes n,, n2, n3, n4, n, of the CMOS integrated circuit under test when each test pattern sequence is applied to the input terminals x1, x2, x3, x4; and the fourth column shows a train of transition signal values which occur on the output terminals z1, z2 of the CMOS integrated circuit under test which result from the transition signal values on these nodes. Signals xe2x80x9cLxe2x80x9d, xe2x80x9cHxe2x80x9d, xe2x80x9cRxe2x80x9d, and xe2x80x9cFxe2x80x9d entered in the second, the third and the fourth column in FIG. 15 represent a normally low level signal  less than xe2x80x9c0xe2x80x9d, xe2x80x9c0xe2x80x9d greater than  (it being understood that a first element within  represents an initial signal value and a second element represents a final signal value), a normally high level signal  less than xe2x80x9c1xe2x80x9d,xe2x80x9c1xe2x80x9d greater than , a signal which rises from a low level to a high level  less than xe2x80x9c0xe2x80x9d, xe2x80x9c1xe2x80x9d greater than  and a signal which falls from a high level to a low level  less than xe2x80x9c1xe2x80x9d, xe2x80x9c0xe2x80x9d greater than , respectively.
Thus each test pattern sequence comprises two test patterns. For example, a test pattern sequence T1=xe2x80x9cLLRxe2x80x9d signifies that x1, x2, x3= less than xe2x80x9c000xe2x80x9d, xe2x80x9c001xe2x80x9d greater than . Numerals entered in parentheses below corresponding signal values represent a transition time of each signal as referenced to the transition time of the signal at the input terminal which is chosen to be 0. For example, R(3) indicates the occurrence of a rising transition signal (xe2x80x9cRxe2x80x9d) at time 3.
The fifth column in FIG. 15 shows a set of delay faults which can be detected during the transient power supply current testing or a fault list when each test pattern sequence is applied to the integrated circuit under test. When a logic gate has a gate delay fault, the output transition time of the logic gate is retarded. Concomitantly, a logic gate which is connected at its input to the output of this faulty logic gate has the timing of a peak of its transient current retarded, whereby the transient power supply current of the integrated circuit under test exhibits an abnormality. In this manner, from an observation to see whether or not an abnormality occurs in the transient power supply current when a test pattern sequence is applied, it is possible to determine whether or not a logic gate which undergoes a switching operation (either a rising or falling transition) in response to the input test pattern sequence is faulty as a logic gate which is connected at its input to the output signal line from the first mentioned logic gate undergoes a switching operation.
By way of example, when a test pattern sequence T2 shown in FIG. 15 is applied to the CMOS integrated circuit under test shown in FIG. 14, there occurs a transition signal F(1) at an output node n1, of a logic gate G1 in the CMOS integrated circuit under test, as shown in FIG. 16. Similarly, transition signals R(2), and F(4) occur at output nodes n2and n3 of logic gates G2 and G3, respectively. As a result of the transition signal R(2) occurring at the node n2 and the transition signal F(4) occurring at the node n3, there occur at the output node n4 of the logic gate G4 a rising transition R at time 4 and a failing transition F at time 6. Thus, the transition signals R(4)-F(6) occur at the node n4. Similarly, the transition signals R(5)-F(7) occur at the output terminal z1 of the output buffer G6. Accordingly, when there exists a slow-to-fall fault in either one of the logic gates G1, G3 and G4 or when there is a slow-to-rise fault in the logic gate G2 or G4, an abnormality in the transient power supply current is observed during the transient power supply current testing which uses the test pattern sequence T2. In other words, the transient power supply current testing which uses the test pattern sequence T2 allows a slow-to-fall fault in either one of the logic gates G1, G3 and G4 and a slow-to-rise fault in either logic gate G2 or G4 to be detected. Thus, a fault list which is determined by the test pattern sequence T2 can be determined as {G1F, G3F, G4F, G2R, G4R} from the described IDDT fault simulation, as indicated in the row of test pattern sequence T2 in FIG. 15. It will be understood that xe2x80x9cFxe2x80x9d and xe2x80x9cRxe2x80x9d in the fault list represent a slow-to-fall fault and a slow-to-rise fault, respectively.
It will be understood from the foregoing that a fault list, in unit of a gate delay fault which are detectable with a test pattern sequence can be prepared.
IDDT fault simulation method (for open faults)
IDDT fault simulation method which prepares a fault list in unit of a open fault will now be described. FIG. 17 shows CMOS integrated circuit under test. This circuit is the same as shown in FIG. 14, but signal lines connecting between input terminals x1, x2, x3, x4 and logic gates as well as signal lines connecting between logic gates are designated by signal lines m1, m2, . . . , m13. It is understood that the signal lines include input and output signal lines as well, and a branch signal line is treated as a distinct signal line. Output signal lines m12 and m13 are connected to output buffers G6, G7 , respectively.
An example of results of IDDT fault simulation which is performed upon the CMOS integrated circuit under test is illustrated in FIG. 18. In FIG. 18, the first column, as counted from the left, represents an identifier for a test pattern sequence; the second column show input signals applied to the input terminals x1, x2, x3, x4 of the CMOS integrated circuit under test; the third column show signals occurring on signal lines m1, m2, . . . , m13 of the CMOS integrated circuit under test when each test pattern sequence is applied thereto; and the fourth column show signals appearing on the output terminals z1, z2 of the CMOS integrated circuit under test when each test pattern sequence is applied thereto. Signals xe2x80x9cLxe2x80x9d, xe2x80x9cHxe2x80x9d, xe2x80x9cRxe2x80x9d and xe2x80x9cFxe2x80x9d remain the same as described above in connection with FIG. 15. For example, a test pattern sequence T=xe2x80x9cRLLLxe2x80x9d signifies that x1,x2,x3,x4= less than xe2x80x9c0000xe2x80x9d, xe2x80x9c1000xe2x80x9d greater than . The fifth column in FIG. 18 show a set of signal lines having open faults which are detectable by the transient power supply current testing when each test pattern sequence is applied to the CMOS integrated circuit under test, or a fault list. When a signal line in the integrated circuit has an open fault, the switching operation of a logic gate which is connected at its input to the faulty signal fine will be retarded, accompanying a change in the transient power supply current waveform of the logic circuit and thus exhibiting an abnormality in the transient power supply current of the integrated circuit under test. Accordingly, from an observation to see whether or not an abnormality occurs in the transient power supply current when a test pattern sequence is applied, it is possible to determine whether or not a signal line which undergoes a switching operation in response to the input test pattern sequence is faulty when a logic gate which is connected at its input to the faulty signal fine undergoes a switching operation.
By way of example, when a test pattern sequence T2 shown in FIG. 18 is applied to a CMOS integrated circuit under test shown in FIG. 17, a transition signal F(0) occurs on a signal lines m1 in the CMOS integrated circuit under test, and transition signals F(1), F(1) and F(1) occur on signal lines m5, m6 and m7, respectively, as shown in FIG. 19. Similarly, transition signals R(2), F(4), F(4), F(4), and R(4)-F(6) occur on signal lines m8, m9, m10,m11 and m12, respectively, and transition signals R(5)-F(7) occur on the output terminal z1. A transition signal F(4) occurs on the signal line m11, but there occurs no transition in the output of the logic gate G5 which is connected at its input to the signal line m11. Accordingly, whenever there is an open fault in either one of the signal lines m1, m5, m6, m7, m8, m9, m10 and m12, there occurs a transition in the output of a logic gate which is connected at its input to such signal line. Accordingly, an abnormality in the transient power supply current is observed during the transient power supply current testing which uses the test pattern sequence T2 if an open fault exists on one of these signal lines. In other words, the transient power supply current testing which uses the test pattern sequence T2 allows the presence of an open fault on the signal lines m1, m5, m6, m7, m8, m9 m10 and m12 to be detected. Thus, a fault list which is obtained with the test pattern sequence T2 is determined by the described IDDT fault simulation to be {m1, m5, m6, m7, m8, m9, m10, m12},as indicated in the row of the test pattern sequence T2 in FIG. 18. The list of open faults is indicated in terms of signal lines on which an open fault is likely to occur.
It will be understood from the foregoing that a fault list, in unit of an open fault, which are detectable with a test pattern sequence can be prepared. IDDT fault simulation method mentioned above is not limited to the detection of open faults on signal lines which connect between logic gates, but is equally applicable to open faults on signal lines which are located within logic gates, by assuming the presence of a fault on a signal line within a logic gate.
IDDT fault simulation method (for path delay faults)
The fault simulation method according to the present invention which prepares a fault list in unit of a path delay fault will now be described. An example of results of a fault simulation which is performed upon the CMOS integrated circuit under test shown in FIG. 14 is illustrated in FIG. 20. In FIG. 20, the first, the second, the third and the fourth column have the same meaning as in FIG. 15. The fifth column show a set of path delay faults (a fault list) which are detectable by the transient power supply current testing when each test pattern sequence is applied to the CMOS integrated circuit under test. When the integrated circuit under test has a path delay fault, an output transition time from the faulty path is retarded. Concomitantly, a change in the output transition time of every or any one of logic gates located on the faulty path causes the timing of a peak in the transient current of a next stage logic gate which receives the output from such logic gate at its input to be retarded, whereby the transient power supply current of the integrated circuit under test exhibits an abnormality. Accordingly, by applying a test pattern sequence to the integrated circuit under test and observing whether or not an abnormality occurs in the transient power supply current of the integrated circuit under test, it is possible to determine whether or not a path delay fault has occurred on any signal propagation path on which every logic gate on this path undergoes a switching operation (either rising or falling transition) in response to the input test pattern sequence.
By way of example, when the test pattern sequence T1 is applied to the CMOS integrated circuit under test as shown in FIG. 14, there occurs a transition signal F(1) at the output node n1 of the logic gate G1 disposed within the CMOS integrated circuit under test, as shown in FIG. 21a. Similarly, signals R(2), R(4) and R(5) occur at the outputs of the logic gates G2, G4 and the output buffer G6, respectively. In this manner, because every logic gate located on a signal propagation path {x1, n1, n2, n4, z1} is subject to a switching operation, it follows that whenever there is a path delay fault on the signal propagation path {x1, n1, n2, n4, z1}, an abnormality in the transient power supply current is observed during the transient power supply current testing which uses the test pattern sequence T1. In other words, the transient power supply current testing which uses the test pattern sequence T1 allows a path delay fault or faults on the signal propagation path {x1, n1, n2, n4, z1} to be detected. In this manner, a detected fault list which can be obtained with the test pattern sequence T1 can be determined by the described fault simulation to be as {{x1, n1, n2, n4, z1}}.
In another example, when a test patterns sequence T2 is applied to the CMOS integrated circuit under test, a transition signal F(1) occurs at the output node n1, of the logic gate G1 within the CMOS integrated circuit under test, and similarly, signals R(2), F(4), R(4)-F(6), R(5)-F(7) occur at the outputs of the logic gates G2, G3, G4 and the output buffer G6, as shown in FIG. 21b. Since every logic gate located on signal propagation paths {x1, n1, n2, n4, z1} and {x1, n1, n3, n4, z1} is subject to a switching operation, if there is a path delay fault on either signal propagation path {x1, n1, n2, n4, z1} or {x1, n1, n3, n4, z1}, an abnormality in the transient power supply current is observed during the transient power supply current testing which uses the test pattern sequence T2. If a path delay time on the signal propagation path {x1, n1, n2, n4, z1} increases, the rising transition R(5) at the output signal line z1 will be retarded, causing a likelihood that a signal transition may not occur at z1. This prevents the path delay time on the signal propagation path {x1, n1, n2, n4, z1} from being determined by the transient power supply current testing, resulting in a failure to detect a path delay fault on the signal propagation path {x1, n1, n2, n4, z1 }. Thus, a path delay fault or faults which can be detected by the transient power supply current testing which uses the test pattern sequence T2 are only those path delay faults which occur on the signal propagation path {x1, n1, n3, n4, z1}. Consequently, a detected fault list which can be determined with the test pattern sequence T2 is determined from the described fault simulation to be {{x1, n1, n3, n4, z1}}.
Generally, when there occur a plurality of transition signals on a signal line in response to a test pattern sequence, a path delay fault which is detectable by the transient power supply current testing is only a path delay fault on a signal transmission path which gives rise to a transition signal on the signal line, that is the same as a last transition signal on the signal line. For example, it may be assumed that transition signals R(a)-F(b)R(c)-F(d)-R(e) occur on a signal line in response to a test pattern sequence. Here it is assumed that transition times a, b, c, d and e are related such that a less than b less than c less than d less than e. In this instance, a signal transmission path which can be tested by the transient power supply current testing is limited to only a path which produce transition signals R(a), R(c) and R(e).
It will be understood from the foregoing that a fault list in unit of a path delay fault which are detectable with a test pattern sequence can be prepared. A signal transmission path having a path delay fault which is registered with a fault list is not limited to those paths which extend from an input terminal to an output terminal of a circuit under test, but may also include a signal transmission path such as {x1, n1} or {x1, n1, m3} in the semiconductor integrated circuit shown in FIG. 14 which does not reach the output terminal.
In addition, the fault simulation method and the fault simulator according to the present invention is not limited to in its application to a CMOS integrated circuit, but is equally applicable to other types of semiconductor integrated circuits.